This study provides detailed information on the effects of particle size, respiratory parameters, route of entry, and constitutional variables on particle deposition efficiency at the various deposition sites in the human respiratory tract. Ferric oxide microspheres tagged with gamma-emitting isotopes are inhaled by human adult volunteer normals. Measurements of particle retention in the head and thorax are made immediately after exposure, and also 24 hours later, using external collimated scintillation detectors within a low-background chamber. Intra-bronchial deposition patterns are studied using gamma-tagged aerosols deposited in hollow bronchial casts, where deposition is measured as a function of branching level. The correspondence of the cast data to the in-vivo situation will be tested by performing cast inhalations on specimens taken from donkeys whose in-vivo deposition patterns have previously been tested with the same inhaled aerosols. The in-vivo and cast data will be used to test predictive mathematical models for lung deposition which have been proposed, and to generate improved models if they are needed.